1. Field of the Invention
The present invention relates to a solid-state imaging device having a plurality of photoelectric conversion elements arranged in a one-dimensional or two-dimensional array above a substrate.
2. Description of Related Art
A one-dimensional image sensor that reads a document in a state almost in contact with the document and in a condition where a read width is set equal to an original document width, a so-called same size and contact type line sensor is widely used for a document scanner of a copying machine or a FAX. In the case of the same size and contact type line sensor, miniaturization of a scanner and reduction in cost are expected since an optical system which forms a document image on an image sensor in a reduced state is not needed.
However, in an actual semiconductor process using a silicon wafer, a plurality of image sensors are used in a connected state because an image sensor having a size larger than a wafer size (currently, maximum 300 mm in diameter) cannot be manufactured. As methods of connecting image sensors, a method of arraying a plurality of packaged image sensors in a zigzag manner in a state where pixels overlap each other and a method of connecting image sensor chips have been proposed as disclosed in The Institute of Television Engineers, Technical Report, ED681, p 49, 1982 and ED812, p 25, 1984.
In the image sensor manufactured by using a current semiconductor process, however, sensitivity variation and black level variation for every pixel occur within a wafer and between wafer lots. For this reason, when a plurality of image sensors are connected, a variation in characteristic of the sensitivity or black level in connecting portions occurs. This generates a streaky fixed pattern noise in a scanned image. Therefore, an additional signal processing circuit for suppressing the fixed pattern noise is needed.
On the other hand, it is also proposed to manufacture a large seamless image sensor at a time using an a-Si (amorphous silicon) process used for a flat panel display, which is not a semiconductor process using a silicon wafer, as disclosed in ‘Electronics’, 1989-7, p 51-p 56. In addition, manufacturing a large image sensor with a-Si is adopted for not only a line sensor but also a large X-ray sensor used in a medical field as disclosed in ‘Applied physics’, volume 73, number 7, p 931-p 934, 2004.
In the case of the a-Si, however, the mobility of electron and hole is low and a variation in electrical characteristic (particularly in Vth) in manufacturing is large compared with a silicon wafer which is crystalline silicon. Accordingly, the a-Si is not suitable for an image sensor for high sensitivity, high-speed reading, and high S/N.
Although a CCD or a CMOS sensor is excellent in terms of high sensitivity and high-speed reading since the CCD or the CMOS sensor is formed of single crystal silicon, it is not possible to manufacture a sensor having a diameter of a silicon wafer, currently, a sensor having a size of maximum 12 inches (300 mm) because a silicon wafer is manufactured as a substrate.
On the other hand, if amorphous silicon is used, a large sensor may be manufactured without a limitation in the substrate size by using a process adopted in manufacturing of a display device. In this case, however, high-speed reading cannot be expected theoretically since the mobility of electron and hole in the amorphous silicon is equal to or smaller than 1/100 of that of single crystal silicon.
As a method of solving the above problems and realizing a large sensor which has high sensitivity and performs high-speed reading, a connection type sensor in which a plurality of CCDs or CMOS sensors are connected has been proposed and used (refer to JP-A-2000-278605, JP-A-2001-42042, JP-A-2003-163796, and JP-A-2003-198813).
A problem of a connection type sensor lies in a connecting method. Although the CCD or CMOS is configured to include a plurality of photodiodes arrayed and a signal readout circuit that is connected to each of the photodiodes and reads a signal charge in a sequential manner, the photodiodes and the signal readout circuit cannot be formed up to the approximate end of a chip surface (since the performance deteriorates due to an influence of the chip end surface). Accordingly, the photodiodes cannot be disposed at equal distances on the basis of a connected result.
For this reason, either a method of performing division imaging by inserting an optical system for dividing an image between an imaging lens and an image sensor or a method of arraying a plurality of imaging devices in parallel so as to overlap each other and performing mixing by signal processing is used.
Furthermore, in the CCD or the CMOS sensor, variations in characteristics for every element, particularly variations in sensitivity characteristic and black level occur. Accordingly, even if elements are arrayed with mechanically high accuracy, a sensitivity difference or a black level difference occurs in a connection portion of the elements, resulting in a fixed pattern noise. Although the fixed pattern noise can be suppressed by a signal processing circuit, the cost increases in the case.
Although the connection type sensor has the high sensitivity and performs high-speed reading as described above, there were disadvantages in that the cost for the connection increased and an imaging optical system was made large. Moreover, in the case of an area sensor, four or more sensor chips arrayed in at least two directions need to be connected at end surfaces in four directions. However, it was almost impossible to array pixels on the end surfaces in the four directions.